Sperm nursery cells, or spermatogonial stem cells, are immature cells that give rise to sperm. Like other stem cells, they live longer than mature cells and generate new cells.

"This advance opens up an exciting range of possibilities for future research, from developing new treatments for male infertility to enhancing the survival of endangered species," said Dr Duane Alexander, director of the National Institute of Child Health and Human Development, which funded the study.

Researchers from the University of Pennsylvania school of veterinary medicine in Philadelphia said they knew the cells would probably work to make sperm in the lab but the precise recipe had eluded researchers.

"In adults, SSCs [spermatogonial stem cells] are the only stem cells that are able to transmit genetic information to subsequent generations," they wrote in their report.

Batches of such cells grown in lab dishes could be genetically engineered and used to make gene-manipulated animals, they said.

Currently the trick is done by inserting new, desired genes into fertilised eggs of very early embryos.

They said researchers could use the new method to put a new gene into a sperm stem cell, grow those cells into mature sperm, and then use all these new genetically modified sperm to father animals with the desired trait.

These animals could pass the new trait on to their offspring.

And it would work to help some infertile men, Brinster said. "This finding is likely to be applicable to humans," he said.

Sperm can be frozen but not indefinitely. Batches of stem cells often grow for very long periods of time.

Boys too young to produce sperm could have such cells saved before undergoing chemotherapy likely to render them infertile, the researchers said.